This length refers to the internuclear separation between the atoms in the element.

Most data are abstracted from reference 1 but some missing values are taken from the primary literature. Lengths do depend upon the allotrope, or form of the element, to some extent. Thus, the C-C length in graphite is 142.6 pm whereas the C-C length in diamond is 154.1 pm. While values are given for the Group 18 gases, these are separations, not bond lengths.